SU107164A1 - Device for the protection of asynchronous electric motors - Google Patents

Description

The subject of the invention is a device for protection of asynchronous electric motors, in particular, mine motors, with the help of a polarized relay connected through a rectifier to a three-phase current adder and switching off the contactor when the motor is overloaded.

In order to use the same polarized relay to prevent the engine from starting while reducing the stator coupling, it is proposed to connect the indicated relay together with the rectifier at one end to the grounded winding of the voltage transformer and the other through the normally closed contactor contact block stator winding.

Obtaining a current-dependent response time characteristic of protection is achieved in the proposed device by applying two opposite-connected windings in the specified polarized relay, in the circuit of one of which is connected a capacitor.

The drawing shows a circuit diagram of the proposed protection device, based on the use of an active capacitive three-phase current adder type

where: Up is the voltage vector at the output of the adder, / A is the phase A vector, / is the phase C vector

Xf VI R - reactive and active resistance.

The specified adder consists of two current transformers CT and TT, resistance and capacitor C ;.

U ,, ,, R,

No. 107164

During the operation of the electric motor (connected to the terminals Mi, MZ, yVIg), a current appears at the output of the adder, which through the rectifier B affects the polarized RP relay. It has two identical windings, one of which is OXX connected in series with capacitor C so that the magnetic fields of the windings are mutually destroyed.

When the voltage is applied to the relay, the capacitor C is charged. After charging the capacitor, the passage of current through the winding, connected in series with the capacitor, is stopped and the relay is activated.

The circuit parameters of the protection apparatus are selected in such a way that, with a motor rated load, the current at the output of the adder is insufficient to operate the RP relay.

In case of a symmetric or asymmetrical overload of the motor, the current at the output of the adder increases, which leads to the operation of the PE relay.

The response time of a relay depends on the magnitude of the overload, decreasing with increasing overload and, conversely, increasing with decreasing overload.

Short-circuit currents cause almost instantaneous protection.

The starting currents of the electric motors act briefly, the capacitor Cg does not have enough time to charge for this time, and they do not trigger the protection.

Any overload (symmetrical and pesymmetrical) of an electromotor-motor leads to an increase in its current, which causes a corresponding increase in current in the PU relay circuit.

The relay YA, work-in, closes its n. about. contact in the circuit of the relay of blocking of the RB, which self-locks along the circuit: secondary winding of the transformer 7J stop button, winding of the relay of RB, block contact of RB-1 of the relay of RB and secondary winding of the transformer T.

The operation of the relay RB leads to the opening n. h contact RB-2 in the circuit of the contactor K's coil, as a result of which it is de-energized and shuts off the electric motor. Simultaneously n. about. The RB-3 and RB-4 contacts close the siren circuit C and the alarm circuit to the central point of the site or station.

After elimination of the malfunction, the button "stop" is pressed, the relay RB is unlocked, and the circuit returns to its original position.

To obtain the same currents in the RP relay with different electric motors, the circuit provides a set of active resistances RZ - RT, which bypass the output of the adder. Each resistance corresponds to a certain power of the motor. Therefore, when the motor is turned on, the switch P must be set to the position corresponding to the power of the connected motor.

According to the scheme 5 / С- / and МК-2 designate auto-start contacts.

The insulation resistance of the motor winding and connecting cable is monitored with the adder disconnected. Protection circuit: secondary transformer winding T, rectifier B, relay RP, rectifier B, resistance R, n. h contact K., terminal of MS connecting cable, place of low insulation resistance, ground, transformer secondary winding T.

Since the summator requires compliance with a specific phase sequence after connecting the motor, the starter must be tested. If there is an immediate shutdown of the electric motor, then it is enough to change two phases, which is easily accomplished if there is a reverse disconnector P.

The proposed scheme, developed for typical electric power 1.7; 2, 5; 5.5; 6.3 and 8.0 kW, can be used to protect electric motors of any power with both phase and short-circuited rotors.

When using it to protect high-power electric motors, it is sufficient to use current transformers designed for the corresponding motor current and replace active resistances RZ - - 7.

Claims (2)

1. A device for protecting asynchronous electric motors with a polarized relay connected via a rectifier to a three-phase current adder and disconnecting the contactor when the electric motor is unloaded, so that in order to use the same relay to prevent the motor from starting when the isolator is lowered stator winding, the relay, together with the rectifier, is connected at its one end to the grounded winding of the voltage transformer, and the other through a normally closed control unit the act of contactor to the stator winding. 2. The device according to claim 1, which differs from the fact that, in order to create a current-dependent characteristic of the response time, the polarized relay has two oppositely connected windings, one of which has a capacitor connected in the circuit. No. 107164 {5k